This invention relates to a printing blanket construction, and more particularly to a metal backed printing blanket construction having a specialized corrosion resistant treatment that is attached to the underside (or reverse) of the metal blanket. The type of blanket referred to herein is used primarily in offset lithographic printing, but may also find utility in other fields of printing.
In offset lithography, a rotary cylinder is covered with a printing plate that normally has a positive image area receptive to oil-based inks and repellent to water and a background area where the opposite is true. The printing plate is rotated so that its surface contacts a second cylinder covered with a rubber-surfaced ink-receptive printing blanket. The ink present on the image surface of the printing plate transfers, or offsets, to the surface of the blanket. Paper or other sheet stock to be printed is then passed between the blanket-covered cylinder and a rigid back-up cylinder to transfer the image from the surface of the blanket to the paper.
One common type of printing blanket is typically manufactured as a flat, fabric sheet with an elastomeric surface that is receptive to ink. Such a blanket is mounted by wrapping it around the blanket cylinder. Various means are used to secure the blanket to the cylinder. Typically, the cylinder has a relatively wide gap or groove in its surface (referred to herein as “the cylinder gap”), running in the axial direction, and the leading and trailing ends of the printing blanket are inserted into the gap and secured by any one of a variety of holding devices. Such devices include reel rods and lock-up mechanisms (see, e.g., U.S. Pat. No. 4,870,901 to Norkus), bar supports (see, e.g., U.S. Pat. No. 4,092,923 to Bollmer) and clamps (see, e.g., U.S. Pat. No. 5,329,853 to Dilling and Stegmeir) adapted to grip the ends of the blanket that are inserted into the cylinder gap. The leading and trailing edges of such blankets are generally reinforced with strips of metal, known as “blanket bars”, to stiffen the blanket edges and to facilitate insertion of the blanket into the holding device inside the cylinder gap (see, e.g., U.S. Pat. No. 4,090,444 to Stearns).
A metal-backed printing blanket typically comprises a base layer of a thin, flat, flexible sheet of metal and a top layer comprising an elastomer such as rubber. Other layers may be sandwiched between the base and top layers, formed of materials such as fabric, after which these multiple layers are laminated together. Such a blanket conventionally has a thickness of about 2 mm, of which about 0.20 mm may be attributed to the thickness of the metal base plate. One configuration of a metal-backed blanket manufactured and sold by KBA (Koenig & Bauer-Albert AG, of Frankenthal, Germany) has a small strip of exposed metal at the leading and trailing edges of the blanket adapted for insertion into the cylinder gap. See, e.g., Puschnerat et al, U.S. Pat. Nos. 5,687,648 and 5,934,194. See also Castelli et al, U.S. Pat. No. 5,749,298.
During the step in which the image is transferred from the plate to the blanket and the step where the image is transferred from the printing blanket to the paper, it is important to have intimate contact between the two contacting surfaces. This is ordinarily achieved by positioning the blanket-covered cylinder and the supporting cylinder it contacts so that there is a fixed interference between the two so that the blanket is compressed throughout the run to a fixed depth, typically approximately 0.002 to 0.006 inches. It is important that this compression be maintained uniformly over the entire surface of the blanket.
Conventionally, this fixed interference is accomplished by inserting one or more thin layers of paper or the like between the blanket and the surface of the cylinder to build up the thickness of the blanket. This process is known as packing a blanket. This process presents problems however in that the packing procedure is time consuming, resulting in down time for the printing equipment. Further, once positioned on the cylinder, the packing paper tends to slide, slip, and/or fold which may render the blanket surface nonuniform and resulting in poor printing results. Further, when a blanket must be replaced, the time consuming packing operation must be repeated for a new blanket.
So-called “no pack” blankets have been developed to provide a fixed interference without the need to pack the blanket. No pack blankets are manufactured to very precise gauges so that one can be installed directly onto a cylinder with the correct amount of interference. These blankets have the advantage of a one-piece construction which requires no positioning of packing paper beneath the blanket. This results in less down time for the printing equipment when an old blanket is removed and replaced with a new blanket.
Such no pack blankets, like most printing blankets, are normally composed of a base material which gives the blanket dimensional stability. Presently most, if not all, commercial printing blankets use woven fabrics for the base material. The base may consist of one or more layers of such fabric. The working surface of the blanket which contacts the ink is typically an elastomeric layer of natural or synthetic rubber which is applied over the base layer or layers. The base layer or layers and working surface are laminated together using suitable adhesives.
In offset lithography as well as other printing operations, the printing plate and blanket cylinders are subject to corrosion and rust because of exposure to inks, water, and chemicals used in cleaning up the machinery. To combat such problems, these cylinders have typically been plated with chrome or nickel, as disclosed in U.S. Pat. No. 5,366,799 issued to Pinkston et al. These metals provide a surface that is not only corrosion resistant, but also ink repellent.
However, such nickel- and chrome-plated cylinders have not worked well in conjunction with no pack blankets. After only short periods of use, nickel is removed from the cylinder surface to such an extent that uncoated steel is exposed. While chrome plating is more resistant to removal than nickel, it too is subject to wear. The areas on the cylinder surface where the plated metal is removed are then subject to rapid corrosion and/or oxidation. Some have speculated that the nickel or chrome is removed by corrosion from chemicals which wick around the edges of the printing blanket. Others have speculated that the metal removal is caused by electrical charges building up from the friction between the blanket and cylinder.
An alternative to using nickel- or chrome-plated cylinders is to coat the printing blanket or the cylinder surface with a plastic adhesive foil, such as polyester. This is done by gluing the adhesive foil to the cylinder's surface, or alternatively, directly to the back of the metal backed blanket. These adhesive foils have the many of the same protective characteristics of the metal plated cylinders, but do not experience the degree of corrosion and oxidation that the metal plates are subject to.
These adhesive foil coatings are not without problems. Exposure to the same inks, water, and chemicals that cause the corrosion/oxidation problems in the metal plated cylinders can cause bubbles to form between the polyester film, and the surface of the cylinder. These solvents penetrate the foil coating from either side of the cylinder, resulting in the bubbling and delamination of the foil coating.
An important goal in offset printing is to increase the operating speeds of printing presses in order to maximize production. However, flaws and imbalances in the printing blanket become magnified as the rotational speed of the blanket cylinder is increased. In particular, high-speed rotation of a cylinder with a cylinder gap can result in undesirable levels of vibration and shock loading. Bubbling and delaminating as described above causes the weight of the cylinder to be unevenly distributed about its axis. The resultant eccentric loading increases vibration during high-speed rotation of the cylinder, to the detriment of print quality. Fabric backed printing blankets are particularly susceptible to the deleterious effects of vibrations during high speed operations, such as slipping and smearing of ink as it is transferred from one surface to another.
Furthermore, high-speed operations increase shock loading, which occurs when the edges of the gap contact the adjoining printing plate. This repetitive impact causes the cylinder and the mounted blanket to bounce, causing the ink to streak and increasing wear on both the blanket and the cylinder.
Thus, it is desirable to create a coating for blanket cylinders that does not experience the drawbacks that are seen with the current plastic adhesive foils, or the metal-plated cylinders. Therefore, there exists a need in the art for a no-pac blanket that can prevent corrosion of the blanket cylinder without resulting in lengthy downtime of the machine, or a drop off in the print quality due to bubble formation.